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Posted on Tue, Apr 5, 2011 : 5:50 a.m.

Story of modern horse's evolution intertwines humans and the ubiquitous plant we call grass

By Kathleen Lundberg

Horses evolved here in North America. Based on fossils collected from the western United States in the 1800s, the orderly progression from a small, dog-like, five toed, leaf-browser (Eohippus, the “Dawn Horse”, now known as Hyracothere) to modern-day Equus has been touted as an example of evolution occurring over thousands to millions of years.

The change from the small, jungle-dwelling Hyracothere to modern Equus reflect adaption to a drier climate with more grassland.

Image courtesy of Wikimedia

It now appears that horses are survivors of a complex family tree with many more side branches and extinct relatives than previously envisioned (note that this link is not for the faint-hearted).

Instead of an orderly sequence of gradual improvements, the horse family lived, changed and died in response to random genetic variation, natural selection, changes in our fickle earthly environment and the co-evolution of plants and people.

Over the past 55 million years, there have been some 30 to 40 branches in the horse family, with a few hundred extinct species along the way.

The five-toed ancestor of the modern horse came to rely on speed to evade its predators in an increasingly dry and open landscape. The middle toe’s fingernail, or claw, developed into a hoof that became bigger and better able to provide traction for fast get-aways. The first and fifth toes became smaller and disappeared.

Horses are related to other three-toed animals that branched off at this juncture: the rhinoceros, tapir and other extinct brethren.

The horse's strategy of fast-breaking speed led to the single toe and highly specialized hoof.

Studying horses’ teeth is an important way in which researchers trace their evolution. Teeth provide clues to what was being eaten and what was growing there and then.

It seems the early equines dined on leaves and fruit typical of a tropical environment. As their world dried out and grasses became available, horses’ teeth grew longer, with adaptations for breaking off and crushing the tougher grass.

Horses graze in a grassy pasture.

Kathy Lundberg | Contributor

Wide grasslands were perfect for long, swift-legged herbivores. Large herds developed and returned the favor to the grasses by eating and destroying trees, providing space and sunshine for the grasses to thrive.

Grass is considered a flowering plant and came along just before the disappearance of dinosaurs. Some 55 million to 65 million years ago, just as little Hydracothere was creeping around, grass was taking hold. Grass is pollinated by wind, freeing its life cycle from the vagaries of insect pollination.

Grass grows fast, can colonize bare ground and tolerates being stepped on or eaten, as its leaves grow from its base. Grass also developed a revolutionary form of photosynthesis, which uses less water, helping survival in drier conditions.

Grass now includes some 10,000 species in its family, providing about 20 percent of the vegetative cover of the world.

Human evolution is also closely tied to the evolution of grasses. Grasses such as wheat, rice, and corn are the most important calorie source for mankind. In fact, the domestication of grass, about 10,000 years ago, marks the start of civilization as we know it. Of course, the domestication of horses 6,000 years ago gave us humans much assistance in growing our grasses and populations.

From the burgeoning grass family tree came an expanded horse family tree, both through adaptive radiation.

The grass family continues to prosper — did you eat your grass today (wheat, rice or corn)?

Of the horse family, only three groups remain on one surviving branch (genus): domesticated horse, zebras and donkeys. Domestic horses split off from zebras and donkeys about 4 million years ago in North America. From here, they spread across the Bering Strait and populated the Old World.

About 14,000 years ago, a capable, clever, two-legged species crossed that same land going in the opposite direction, spreading into the New World. As a result of multiple factors, probably including climate change, possibly including hunting by these early humans or infection by diseases they brought with them to the New World, or even due to a large meteor detonating in the atmosphere, Equus became extinct in the Americas.

As recently as the blink of an evolutionary eye, we humans have re-introduced the sound of hoof beats to the Americas. The story of horses, people and grass continues to unfold.

Kathy Lundberg is a regular contributor on AnnArbor.com's pet section where she addresses topics regarding horses. She is the owner of Scio Church Stables. She welcomes your contact via email.

Comments

julieswhimsies

Wed, Apr 13, 2011 : 1:09 a.m.

The most destructive animal on the planet? Human beings!

REBBAPRAGADA

Thu, Apr 7, 2011 : 3:09 a.m.

Firstly, man does not eat grass. Man consumes seeds and grains and the juice from the stems of grass like Sugarcane. The words have to be carefully chosen. It is the protoplasm, the living substance or living matter which has this ability called Nutrition. It is protoplasm which acquires molecules and matter from its environment. Bacteria, animal cells, plant cells, and the cells of the human orgainsm all exist by deriving energy from an external source. It is interesting to note that this protoplasm has not changed over billions of years. Earth has witnessed several major, and minor extinction events during its long past. The climatic changes, the environmental changes could not change the nature of protoplasm. It just remains unchanged. All these animals which belong to the modern horse family had/have the same kind of protoplasm which acquires its energy from its environment. I would like to hear about any scientific evidence that the climatic or other changes could change the protoplasm.

Kathleen Lundberg

Tue, Apr 5, 2011 : 7:40 p.m.

RK brings up an interesting perspective on horse evolution. Equus was once a natural part of the multi-species habitat on Earth. Humans selected the horse from the wild to eat and to extract labor from. Due to its strength and tractability horses transported us, built our buildings, fought our wars, grew and processed our food, and became extinct in their natural environments. In an increasingly human world, our work is now done largely by machines. Horses are relegated in RK's mind to "expensive and destructive pets".
The topic of garlic mustard (<a href="http://www.nps.gov/plants/alien/fact/alpe1.htm)" rel='nofollow'>http://www.nps.gov/plants/alien/fact/alpe1.htm)</a> is related to the process of evolution, although not directly to horses. Garlic mustard, an herb native to Europe, was introduced to the US by humans in the late 1800's. It is highly invasive, outcompeting native plants for sunlight and soil, wreaking havoc on native plant species and the animal that depend on them. It can be self-pollinating and does not need a support crew of insects to help it multiply. In its native habitat, garlic mustard is found in disturbed terrain – along roadsides and fencerows. In North America, it expands into a wider range of habitat (<a href="http://www.nwcb.wa.gov/weed_info/Written_findings/Alliaria_petiolata.html)" rel='nofollow'>http://www.nwcb.wa.gov/weed_info/Written_findings/Alliaria_petiolata.html)</a> including sunny/shady, urban/rural, along forests/rivers <a href="http://www.ipm.msu.edu/garlicmustard.htm" rel='nofollow'>http://www.ipm.msu.edu/garlicmustard.htm</a>. Once established, it is very difficult to eradicate (<a href="http://www.invasiveplants.net/plants/garlicmustard.html)" rel='nofollow'>http://www.invasiveplants.net/plants/garlicmustard.html)</a>. Garlic mustard is a big problem.
The third topic RK raises is how common space is shared between persons with diverse interests - an ongoing discussion with many voices. Michigan has dedicated horse trails (<a href="http://www.michigan.gov/dnr/0,1607,7-153-10365_16839_54521-119924--,00.html)," rel='nofollow'>http://www.michigan.gov/dnr/0,1607,7-153-10365_16839_54521-119924--,00.html),</a> and trails that are shared. Follow these links to learn more (<a href="http://www.michigan.gov/dnr/0,1607,7-153-10365_16839_54521---,00.html)" rel='nofollow'>http://www.michigan.gov/dnr/0,1607,7-153-10365_16839_54521---,00.html)</a>.

Rork Kuick

Thu, Apr 7, 2011 : 12:12 p.m.

Does nothing to argue that the destruction caused on the public land is sufficiently offset by the folks benefiting. Shall we allow motorized dirt bikes out there - probably more people would like that than bridle trails. Sharing is nice, but there have to be some limits on the damage.
As a tiny detail, I think every organism that became extinct did so in their &quot;natural environment&quot;. The fact remains these are human imports. There still are horses used for purposes other than human recreation, but I've not seen those on the public lands, with rare exceptions far from here (parks in Montana, Idaho).

julieswhimsies

Thu, Apr 7, 2011 : 12:39 a.m.

Great, well-researched come-back! :P

REBBAPRAGADA

Tue, Apr 5, 2011 : 3:55 p.m.

I like the story and I would be pleased if you share this secret about teaching wisdom to the cells of an organism. If it is possible, we could all teach our cells some wisdom and make them adapt to changing climate and environment. The aging process robs the man of his intellectual abilities. Man, known as Homo sapiens sapiens has arrived on this planet Earth in very recent times and he will take time to learn about his Creator.

Rork Kuick

Tue, Apr 5, 2011 : 12:16 p.m.

Visit the horse trails of Waterloo and Pinckney Recreation areas, and see horrible erosion, increased invasive alien plants, and tremendous unwillingness to stay on the designated trails with these expensive and destructive pets. Of course almost no one walks these areas cause it is so unpleasant compared to areas where horses are forbidden. I see them mostly when volunteering trying to control things like well fertilized garlic mustard - I'm talking waist high plants. I think the benefit (to a few) is not worth the cost.